1. Field of the Invention
The present invention relates to a digital signal modulation method capable of accurately performing lower component suppression control for suppressing a lower component in a recording code series when converting data of a digital information signal consisting of p (p is a positive integer) bits into a code word consisting of q (q is an integer larger than p) bits and recording it on a high-density storage medium such as an optical disk, a magnetic disk and a magnetic tape by using a code series satisfying a predetermined run-length limited rule, a digital signal modulation apparatus, a storage medium, a transmission apparatus and a program, and to an encoding method used when recording/reproducing data to/from a storage medium or the like, an encoding apparatus, a storage medium, and a program for realizing the encoding method.
2. Description of the Related Art
As a digital signal modulation mode used for recording a digital information signal on a digital storage medium, a modulation mode with less direct-current signal components included in a modulated signal has been developed, and a digital signal recording apparatus employing this modulation mode has come into practical use.
There is an NRZ (Non Return to Zero) mode as a basic digital modulation mode. In this mode, however, a direct-current component is generated when digital-modulating a digital information signal in which 0 or 1 longly continues, and this direct-current component can not be used in, for example, magnetic recording which can not reproduce a direct-current signal. As a countermeasure, a modulation method such as a mirror code or a mirror square code which does not produce a large quantity of direct-current components has-been developed.
The mirror code is a technique which generates a digital modulation signal by a combination of T, 1.5T and 2T with respect to a bit interval T, but a small amount of direct-current components remains in the modulation signal. The mirror square code is a digital modulation mode which further reduces the direct-current signal components by adaptively selecting one which generates a smaller amount of the direct-current component from inversion or non-inversion with 1 at the center when 1 longly continues in an information signal.
In the mirror square code which reduces its direct-current signal components, a minimum bit inversion interval is T and a maximum bit inversion interval is not less than 3T. Therefore, a ratio of a maximum frequency component and a minimum signal frequency of a digital modulation signal is a value not less than 3, and a signal in such a frequency band is recorded on a storage medium.
On the other hand, in the information-oriented society realized by transmission and recording of digital information signals, both a number of types of information to be processed and a quantity of information are increasing, and materialization of an efficient digital modulation mode for effectively transmitting and recording digital information signals which tend to increase has been long-awaited.
The efficient digital modulation mode enables a larger quantity of digital information signals to be recorded on a digital signal storage medium having a restricted recording frequency capacity by performing digital modulation using modulation signal frequency components which is as low as possible.
Further, although the digital modulation signal recorded on the digital signal storage medium is a low direct-current component because of a magnetic reproduction characteristic in case of magnetic recording, reduction in level of the direct-current component included in the digital modulation signal is demanded in order to perform tracking control which obtains a digital modulation signal while tracking directly above a recording track even in case of a discoid high-density storage medium such as an optical disk.
In this manner, the characteristic of the digital modulation signal in the present day lies in that a frequency of the modulation signal is a low frequency band, and it is often the case that the direct-current frequency component is obtained as a signal which is as low as possible. Furthermore, a technique for conversion encoding is often used in order to obtain such a modulation signal.
The conversion encoding is a method such as 8–10 block conversion which encodes 256 types of information signals each consisting of, e.g., eight bits by selecting 256 types of codes having a number of 1 and that of 0 being substantially equal to each other and a smaller quantity of direct-components from 1024 codes each of which can be represented by 10 bits, and this method is also adopted in DAT (Digital Audio Tape recorder).
Moreover, although a CD (Compact Disc) adopts an EFM (Eight to Fourteen Modulation) mode which converts an eight-bit signal into a 14-bit signal series, the EFM modulation mode uses an encoded series in which a redundant bit used for suppressing the direct-current component is set.
In addition, since the encoding efficiency can be further increased as long as a digital conversion signal in which the direct-current component is suppressed can be generated without setting such a redundant bit, there has been developed a method which reduces the direct-current component by applying DSV (Digital Sum Value) control on a recording code series to be recorded on a high-density information recording medium.
Suppression of the digital modulation signal lower component in a high-density discoid optical storage medium typified by the optical disk aims at minimization of adverse affect on performance for executing high-speed servo control by a lower component of the recording signal series which has been mixed in a focus servo signal or the like as well as a tracking servo signal, and also reduction in undesirable servo control operations such as an erroneous control operation by which a reference level for signal detection fluctuates when detecting an error signal for servo.
As to reduction in the signal component applied to such undesirable servo control, for example, the DVD Forum which operates for DVD (Digital versatile Disc) standardization expressly provides a method of DSV control in the physical specification of a DVD-ROM.
Meanwhile, the DSV control method expressly provided in the DVD-ROM physical specification reduces generation of an undesirable error signal by decreasing an absolute value of a DSV value at the point in time of DSV control. Although the lower component in the digital modulation signal can be reduced, there has been demanded establishment of a technique for generating the digital modulation signal for effecting high-performance DSV control which does not generate the lower component at any other point in time than DSV control.
As to such a method for effecting DSV control by detecting a peak voltage value of the DSV signal, although Japanese patent application laid-open No. 2000-295109 “Digital Modulation Apparatus” is disclosed, its DSV control method optimizes the DSV control by using a data scrambling technique, and it can not be applied to, e.g., a digital modulation mode which is based on block conversion using no data scrambling technique.